Screenshot: Tzolk'in (0.12)

Disk Clock shows the Tzolk’in, and I think it’s a rather nice way to view the interlocking cycles.

The mesoamerican cultures had a fascinating calendar system based on interlocking cycles of 13 and 20. (Tzolk’in to the Maya, Tonalpohualli to the Aztec, and probably other names to other cultures.) Unfortunately, this only accounts for 260 days, so it was usually combined with a solar year counting system based on similar principles, but using a more traditional month/day arrangement. (The Haab) The combination of dates between the two systems repeated every 52 years, which became a time of major celebration.